The present invention relates generally to bandwidth compression techniques for digital audio signals, and more particularly to a variable transmission rate communication system for a network having a cell loss probability such as an asynchronous transfer mode (ATM) network.
Adaptive differential pulse-code modulation (ADPCM) technique is known as a practical way of bandwidth compression and has been extensively used in digital communications. According to this method, input discrete speech samples are buffered to form a block of N speech samples each. All samples of each block are linearly transformed into a group of transform coefficients based on a linear transform. These transform coefficients are then adaptively quantized independently and transmitted. At the receiver, the quantized signals are adaptively dequantized according to a supplementary signal that is derived in a manner inverse to that of the transmitter, and an inverse transform is taken to obtain the corresponding block of reconstructed audio samples.
Another bandwidth compression technique that is attractive for audio signals is the adaptive transform coding scheme (ATC) as shown and described in "Approaches to Adaptive Transform Speech Coding at Low Bit Rates", Rainer Zelinski et al, IEEE Transactions on ASSP, Vol. 27, No. 1, 1979, pages 89-95. A further approach is described in "A Musicam Source Codec for Digital Audio Broadcasting and Storage", Y. F. Dehery et al, Proceedings of ICASSP '91, Vo.. 5, 1991, pages 3605-3608, in which digital audio samples are divided into frequency subbands and each subband is adaptively quantized according to a bit assignment signal.
A technique for ensuring the quality of transmission against lost packets is described in "Embedded DPCM for Variable Bit Rate Transform", D. J. Goodman, IEEE Transactions on Communications, Vol. COM-28, No. 7, July 1980, pages 1040-1046. According to this approach, when least significant bits are discarded by a channel, and the decoder inserts all-zero filler bits to the lost bits prior to decoding.
However, the ATC technique is superior to the DPCM technique in terms of bandwidth compression, and it has been desired to ensure against lost cells when the bandwidth compression technique is used for ATM networks.